According to the theory of solution crystallization, the purity of a crystalline product relies on the crystal size distribution to a large extent. Particularly, the purity of a crystalline product is greatly influenced by the fraction of fines. An increase in the fines fraction inevitably results in an increase in the crystal surface area per unit weight. This not only increases the amount of impure mother liquor entrained by the crystals, but also diminishes the benefit of filtering and washing operations. Consequently, it is difficult to obtain a high purity product in one stage crystallization and generally multi-stage crystallization is needed, causing unnecessary waste. Many studies support this finding. Therefore, in order to improve the product purity or obtain the greatest possible benifit from washing and filtering operations, it would be desirable to obtain a crystalline product with a large and uniform crystal size in the crystallization process.
In a continuous or batch crystallizer, crystal particles are formed as a result of the growth of individual crystal nuclei. In a fixed amount of slurry, if the number of the crystal nuclei is too large, the crystal size of the final product must be small since a limited mount of solute is distributed over excessive crystal nuclei. Therefore, it is necessary to destruct the excessive crystal nuclei in time. An effective destruction of fines is beneficial not only for increasing the mean crystal size of the product, but also for raising the crystal growth rate, because in a crystallizer with a fines destruction system the degree of supersaturation may be somewhat increased, and hence both the crystal growth rate and the production capacity of the crystallizer are improved.
The existing methods for destructing fines are the setting method based on elutriation theory and the size fractionation method, for example, see the following reference:
Randolph, A. D., Larson, M. A. Theory of Particulate Processes, Academic Press, New York, 1988; Nyrlt, J., Industrial crystallization. The state of Art; VCH, Weinheim, 1982; U.S. Pat. Nos. 3,873,275; 5,124,265; Qing Xu-huai and Tan Qiu, Industrial Crystallization, Chemical Industry Press, Beijing, 1985. The settling method requires an annular baffle set in the crystallizer to form a settling zone. However, the presence of the settling zone not only increases the complicacy of the crystallizer structure, but also reduces the effective volume and the production capacity of the crystallizer. Moreover, it is difficult to apply the settling method to a crystallization processes which is carried out under vacuum, pressure or by evaporation. In brief, the settling method is troublesome in operation and poor in adaptability. On the other hand, the size fractination method requires complicated equipments and demonstrates poor suitability. In addition, both the above methods are essentially unsuitable to those systems in which the density difference between solid and liquid is small. PA1 A. two filters are installed in the crystallizer; PA1 B. a part of the slurry in the crystallizer is filtered through the first filter using circulating pump, and a slurry containing fines, substantially all of which are of a size smaller than the fines cut size, is fed to at least one fines destructor in which they are dissolved, and then the solution in which the fines have been destructed is fed back to the crystallizer through the second filter for further crystallization; PA1 C. after a switching interval, a part of the slurry in the crystallizer is filtered through the second filter using a circulating pump, a slurry containing fines, substantially all of which are of a size smaller than the fines cut size, is fed to at least one fines destructor in which they are dissolved, and then the solution in which the fines have been destructed is fed back to the crystallizer through the first filter for further crystallization, thereby washing the first filter; and PA1 D. after a switching interval, repeating procedure B and thereby washing the second filter, and continuing to alternately repeat the procedures B and C to carry out the process of destructing fines continuously.